This project represents a study of cellular kinetics and lethality in four different in vitro cell lines, comparing continuous x-irradiation with the effects of fractionated treatments as practised in cancer therapy. The effects of combined continuous irradiation and mild hyperthermia (39 degrees C - 42.5 degrees C) are also being investigated The cell lines used in the experiments are V79 Chinese hamster, S3 HeLa, Morris hepatoma and a cell line originated from C3H mammary tumor. These cell lines differ considerably in their growth rates as well as their response to acute exposures.